Research press release


Nature Geoscience

Geoscience: Irrigation intensifies humidity-associated heat stress in India

インドでは、灌漑によって湿度が高くなり、これが、最大4600万人が影響を受ける「湿熱ストレス」を増大させていることを報告する論文が、Nature Geoscience に掲載される。熱ストレスは、ヒトの体が自らを冷やすことができないときに生じ、環境の温度が高いことだけで生じる場合(乾熱ストレス)と、温度と湿度が高いことで生じる場合(湿熱ストレス)がある。後者は、インドでの死亡率と密接な関連がある。


今回、Vimal Mishraたちの研究チームは、乾熱ストレスと湿熱ストレスの両方に対する灌漑の影響を、現場での観測と人工衛星による観測、および最新の数値シミュレーションを用いて分析した。その結果、灌漑は地表の温度を下げるが、大気の最下層(大気境界層)の高度を低下させることにより、地表の湿度を顕著に上昇させることが明らかになった。その結果、灌漑は乾熱ストレスを軽減するが、湿熱ストレスを強化する。


Irrigation in India is enhancing humidity, which is in turn increasing ‘moist heat stress’ experienced by up to 46 million people, according to a study published in Nature Geoscience. Heat stress occurs when the human body cannot cool itself, and can result from high environmental temperatures alone (dry heat stress) or from high temperatures with humidity (moist heat stress) — the latter is more closely linked to human mortality in India.

Ongoing human-induced climate change is expected to intensify temperature extremes in India that could exacerbate heat stress, especially among those working outdoors. In addition, India’s irrigation area has doubled in recent decades. Cooling due to water evaporating from irrigation systems might partially counteract dry heat stress, although associated changes in humidity that could affect moist heat stress are poorly understood.

Vimal Mishra and colleagues analysed the influence of irrigation on both dry and moist heat stress using a variety of in situ and satellite observations, together with state-of-the-art numerical simulations. They found that although irrigation causes land surface cooling, it also leads to substantially higher surface humidity by reducing the height of the lowest part of the atmosphere, known as the planetary boundary layer. As a result, irrigation mitigates dry heat stress, but enhances moist heat stress.

The authors conclude that recent intensification of irrigation practices in India has enhanced moist heat stress, and the associated risks to human health, across the local region and neighbouring countries of Pakistan and Afghanistan. Similar impacts from irrigation are also expected in other regions with similar semi-arid and monsoonal climates.

doi: 10.1038/s41561-020-00650-8


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